1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface emitting laser (VECSEL) device, and more particularly, to a VECSEL having an improved structure so that a gain efficiency can increase in quantum well layers and/or a lasing efficiency can be improved in various embodiments.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) has a high coupling efficiency because it generates single longitudinal mode lasing in a narrow spectrum and also at a narrow projection angle. Furthermore, the VCSEL can be easily monolithic-integrated into other devices due to its structure, and thus, it is suitable for a pump laser diode (LD).
However, the area of a lasing region should be 10 μm or smaller for single lateral mode lasing, and even in this case, the single lateral mode may be changed into multi-mode due to a thermal lens effect according to an optical output increase. Therefore, the maximum output of single lateral mode lasing is generally 5 mK or less.
To retain the advantages of the VCSEL while increasing high emission power, a vertical external cavity surface emitting laser (VECSEL) has been developed. The VECSEL has a gain region that is increased by replacing an upper distributed Bragg reflector (DBR) with an external mirror so that the VECSEL can generate lasing at a high emission power of 100 mW or more. Because it is difficult to obtain sufficient gain in a surface emitting layer since the surface emitting laser has a smaller gain volume than that of an edge emitting laser, a VECSEL device of periodic gain structure, in which quantum wells are periodically disposed, has been developed. In addition, since there is a limitation to perform carrier injection evenly onto a larger area with an electric pumping operation, an optical pumping type VECSEL device has been developed for obtaining high output power.
FIG. 1 is a schematic cross-sectional view of a conventional 920 nm VECSEL device.
Referring to FIG. 1, the conventional VECSEL includes a substrate 10, and a DBR mirror 20, a multiple quantum well (MQW) active area 30, and a capping layer 52 that are sequentially formed on the substrate 10, an optical pump 60 supplying a pump beam and an external cavity mirror 70 formed corresponding to the DBR mirror 20 at an external portion. The DBR mirror 20 includes a plurality of low refractive layers 20a and a plurality of high refractive layers 20b that are repeatedly stacked, and the MQW active layer 30 includes a plurality of GaAs barrier layers 30a and a plurality of InGaAs quantum well (QW) layers 30b that are repeatedly stacked.
FIG. 2 is a schematic energy band diagram of the VECSEL device of FIG. 1, and FIG. 3 is a graph illustrating photoluminescence (PL) intensity according to an increase of pumping power in the VECSEL of FIG. 1.
Referring to FIGS. 2 and 3, when the pumping power increases, recombination of electrons and holes is generated more at the barrier area than in the InGaAs QW layers. Therefore, a noise beam is generated with a laser beam of 920 nm wavelength that is meant to be obtained from the VCESEL device, and thus, lasing efficiency of the 920 nm VECSEL device may be degraded. Therefore, a VECSEL device having a structure capable of improving the lasing efficiency is required.